1. Field of the Invention
This invention generally relates to a method and an apparatus for reproducing information from a recording medium. This invention particularly relates to a method and an apparatus for performing automatic or manual adjustments to the frequency response of a signal transmission system, and a gain, a balance, and an offset about a tracking error signal or a focusing error signal with respect to a disc-shaped removal recording medium, and reproducing information therefrom.
2. Description of the Related Art
A typical optical-disc drive apparatus includes an optical head for optically recording and reproducing information on and from an optical disc through the use of a writing laser beam applied to the optical disc and a reading laser beam returned therefrom. The drive apparatus performs tracking control and focusing control of the optical head relative to the optical disc to accurately write and read data into and from the optical disc. The drive apparatus includes servo control circuits for adjusting the optical head to implement the tracking control and the focusing control.
In general, during the recording of information on a recordable optical disc, the power of a writing laser beam focused on the optical disc is adjusted to a level specified by the optical disc. During the reproduction of information from the optical disc, a reading laser beam is optimized by the following steps. Gains related to a tracking error signal and a focusing error signal are changed. Each time the gain change is performed, offsets about the tracking error signal and the focusing error signal are adjusted. Furthermore, balances about the tracking error signal and the focusing error signal are adjusted. The gains, the offsets, and the balances about the tracking error signal and the focusing error signal are set to optimum values.
A physical change in use environments such as an ageing-based change, a temperature change, a humidity change, or a power-supply-voltage change causes the gains, the offsets, and the balances to deviate from the optimum values. In this case, it is necessary to readjust the gains, the offsets, and the balances.
Japanese patent application publication number 8-203107/1996 discloses an apparatus for recording and reproducing information on and from an optical disc. The apparatus in Japanese application 8-203107/1996 includes an optical head for applying a laser beam to the optical disc. The apparatus detects the current value of a physical condition of the apparatus or an optical disc, and compares the current detected value with a reference value or the immediately-preceding detected value of the physical condition. The apparatus decides whether or not the optical head is in a stand-by state or an active state with respect to the optical disc. In the case where the current detected value of the physical condition exceeds the reference value or differs from the immediately-preceding detected value thereof by a prescribed value or more, when the optical head is in its stand-by state, the apparatus optimizes at least one of the tracking control and the focusing control of the laser beam with respect to the optical disc.
In the apparatus of Japanese application 8-203107/1996, at least one of an offset, a balance, and a gain about a tracking error signal or a focusing error signal is readjusted so as to follow a variation in the physical condition of the optical disc. Accordingly, the tracking control or the focusing control remains optimized while the physical condition of the optical disc varies.
Removal recording discs include DVDs (digital versatile discs) which are of various types such as a DVD-Video, a DVD-ROM, a DVD-R, a DVD-RW, a DVD-RAM, a DVD+R, a DVD+RW, and an HD-DVD. These DVDs are prescribed by the different standards, respectively. There are optical-disc drive apparatuses which can handle DVDs of plural types. An ideal super multi-drive apparatus can handle DVDs of all the types. When being loaded with a DVD, such a super multi-drive apparatus detects the type of the DVD and adjusts servo and signal-transmission systems therein to states optimum for the detected DVD type. After the DVD type detection and the adjustment of the servo and signal-transmission system are completed, the super multi-drive apparatus starts to, for example, play back the DVD. In general, the DVD type detection and the adjustment of the servo and signal-transmission system take a long time. Accordingly, there will be a long wait until the start of the playback of the DVD.
Some of commercially available DVDs have physical characteristics out of the standards. In some cases, such DVDs can not be played back by normal drive apparatuses.
Japanese patent application publication number 10-11774/1998 discloses an apparatus for reproducing information from an optical disc. The apparatus in Japanese application 10-11774/1998 includes a motor for rotating the optical disc, and an optical head for applying a laser beam to the optical disc. The optical head has sensors for receiving laser light returned from the optical disc. A tracking error signal is generated by calculations using the output signals from the sensors. The optical head is controlled in response to the tracking error signal so as to be properly located with respect to the track on the optical disc. A focusing error signal is generated by calculations using the output signals from the sensors. A focusing servo control device controls the optical head in response to the focusing error signal so as to properly locate the optical head in the focusing direction with respect to the optical disc.
In the apparatus of Japanese application 10-11774/1998, the optical head converts received laser light from the optical disc into an electric signal and outputs the electric signal. A demodulator subjects the output signal from the optical head to demodulation, thereby recovering original information therefrom and generating a reproduced signal. The focusing servo control device is in a control loop having an adjustable gain. A disturbance signal of a specified frequency is applied to the control loop. Signals occurring in the control loop before and after the application of the disturbance signal are compared. The gain of the control loop is optimized in response to the result of the signal comparison. A condition of the reproduced signal is measured synchronously with the disturbance signal. The function of the focusing servo control device to properly locate the optical head in the focusing direction is optimized in response to the measured condition of the reproduced signal.